1. Field of the Invention
The invention relates to a process for preparing highly viscous organopolysiloxanes, and to highly viscous organopolysiloxanes prepared thereby.
2. Background Art
For the preparation of polysiloxanes having high viscosity, a number of methods exist, based predominately on condensation reactions. For instance, a standard method is to prepare silanol-functional, highly viscous polysiloxanes from low-viscosity hydrolyzates of chlorosilanes, usually dimethyldichlorosilane, by condensing them at relatively high temperature, usually under the action of acidic catalysts, and liberating water in the process.
In a similar manner, silanol-functional polysiloxanes can be condensed with methoxysilanes to liberate methanol. For this purpose, catalysts and relatively high temperatures are usually required in order to achieve industrially acceptable conversion rates. For instance, the standard process for preparing typical commercial amino-functional siloxanes is the base-catalyzed condensation of aminoalkyl methoxysilanes with a short-chain hydrolyzate of dimethyldichlorosilane at elevated temperature.
In view of the reaction temperatures of 100° C. and higher, and a reaction time of over several hours, a process for preparing highly viscous polysiloxanes is desirable which is either rapidly complete or proceeds at low temperature such as room temperature, or both.
DE-A 2500020 describes a process for preparing aminosiloxanes in which silanol-terminated polysiloxanes are reacted with a-aminosilanes which bear an alkoxy group. The reaction proceeds at moderate temperatures with elimination of alcohol. However, it is only possible with this technique to prepare comparatively unstable α-aminosiloxanes and also only in difunctional telechelic form.
Highly viscous polysiloxanes can also be obtained by polyaddition reactions, as described in U.S. Pat. Nos. 5,241,034 and 6,252,100. EP-A 874 017 and U.S. Pat. No. 6,451,909 disclose polyaddition reactions in emulsion for the preparation of highly viscous polysiloxanes. However, it is common to all of these polyaddition methods that metal catalysts are required for the progress of the reaction, and these are often undesired. In the presence of N-containing sil(ox)anes, catalyst inhibition also occurs, so that it is barely possible, if at all, to carry out an efficiently catalyzed polyaddition.